The present invention relates to a method for adjusting differential thermal expansion between an electrical socket and a circuit board having different coefficients of thermal expansion (CTE).
With the rapid development of electronic instruments, a variety of corresponding electrical components are successively developed to support the technological advances gained by the electronic instruments. For a modularized central processing unit (CPU) of a computer, an electrical socket is mounted on a circuit board to firmly assemble the CPU thereto. Such electrical connector is arranged with a number of conductive contacts in arrays. One end of each contact extends from one side of the socket for electrically contacting with corresponding conductive sections of the CPU, while the other end of each contact extends from the other side of the socket for electrically connecting with the circuit board.
Through hole technology is often employed for attaching an electrical socket to a circuit board. One end of each contact of the electrical socket extends into a corresponding through hole of the circuit board. However, the through holes of the circuit board should be arranged in arrays corresponding to the contacts of the electrical socket thereby occupying space on both sides of the circuit board. Such a procedure complicates manufacture of the circuit board and does not promote an efficient use of space thereon, especially when the arrangement of the contacts of the electrical socket is dense.
Therefore, a ball grid array (BGA) type modularized CPU package has been developed, which directly solders a number of solder balls to an electrical socket for electrical connection with a circuit board. The corresponding conductive contacts of the electrical socket each form two opposite ends for respectively engaging with the solder balls and disc-shaped contact pads formed on a surface of the circuit board. The solder balls are then heated to reach a melting point thereof whereby the contacts are soldered to the corresponding contact pads.
Due to new applications and configuration designation requirements, an insulative housing of the electrical socket and the circuit board should be constructed from different material with different coefficients of thermal expansion (CTE). The difference in CTE will result in different thermal expansions thereof which may create a fracture stress exerted on the solder balls attached to the contacts of the socket thereby misaligning some of the contacts of the socket with the corresponding contact pads of the circuit board during the heating process.
Thus, if the solder balls can not be properly deposited on the contact pads, the poor soldering quality and inadequate electrical reliability may result. In serious cases, a short circuit may be formed between the electrical socket and the circuit board. Furthermore, a significant fracture stress acting on the solder balls between the contacts of the socket and the contact pads of the circuit board may result in the contacts becoming disconnected from the socket.